Maintaining the correct humidity in your grow room is a critical element of successful indoor growing.

Temperature & Humidity

Humidity levels and temperatures are closely related. Relative humidity (RH), often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity at the same temperature. Importantly, warm air holds more water vapor than cold air. Check out a VPD chart to help visualise the relationship between temperature and humidity.

Low Humidity

If your room is too dry (low humidity) your plants will drink more which can lead to nutrient burn. Low humidity can also lead plants to become dehydrated which can stagnate growth and cause the stomata to close.

When humidity is too low, an evaporative cooler or humidifier can increase humidity levels. Some growers also spray and mist their plants to increase humidity or add open water sources to their room (think trays of water).

High Humidity

Excessive humidity will reduce the plant’s ability to process CO2. This is because overly moist air suffocates the stomata, reducing its ability to collect the CO2 needed for vigorous growth. In addition to this, moist air reduces the plant’s ability to transpire. Transpiration is the movement of water from the roots to leaf surface – driven by evaporation. At high humidity, evaporation is low, so transpiration slows down. As a result of this, growth is adversely affected.

When humidity is too high, consider extra ventilation or a dehumidifier. Another way to reduce humidity is to water your plants at the very start of the light period.

Humidity Swings

Avoid large swings in humidity. Plants prefer relatively stable environments. Similar to when humidity is too high or low, inconsistent humidity levels can cause your plants to display signs of stress. When transitioning humidity between growth stages, do it gradually.

Optimal Humidity

So what is the optimal humidity in a grow room? This depends on the growth stage of your plants.

• Clones and seedlings: 80%. Seedlings and clones like high humidity levels because they don’t have a root system established yet. The high humidity levels allow moisture to be absorbed through the leaves.
• Grow stage: 60%-80%. During the grow phase, humidity levels can be lowered by 5% each week as the plant develops its root system.
• Bloom stage: 50%-60%. This stage requires the lowest humidity levels. DO NOT allow your humidity to go above 60% in this stage.
• Some growers will also reduce humidity further during late bloom (40-50%).

Measuring and Monitoring Humidity

A hygrometer is a useful tool for measuring humidity. A digital hygrometer that measures high and lows is preferable. Check out our post Temps & Humidity: A Guide to learn more.